This invention relates to an apparatus for detecting the passage of multiple superposed sheets along a feed path. The invention has application, for example, to an apparatus for detecting the passage of superposed currency notes in an automated teller machine (ATM).
In a cash dispensing mechanism of an ATM, it is important to provide a simple and reliable means for detecting when a currency note has become superposed on another in a path of travel from a currency supply means to a note exit slot, since such superpositioning may produce an undesirable result such as the dispensing of an excessive amount of money. For convenience, two or more sheets or notes which have become disposed in a superposed relationship will hereinafter be referred to as a multiple sheet or a multiple note.
A known system for detecting multiple sheets includes a sensing station in a feed path for the sheets. The sensing station includes a light emitter and an optical sensor arranged to sense light from the emitter transmitted through an item comprising a single or multiple sheet present at the sensing station. The optical sensor provides an output voltage whose magnitude is dependent on the intensity of the transmitted light received by the optical sensor. The output voltage of the optical sensors is applied to a processing circuit, and on the basis of this voltage the processing circuit provides an output indicative of whether a single sheet or a multiple sheet is present at the sensing station.
With this known system problems have been experienced in reliably distinguishing between a single bank note and a double bank note, and between a single clean bank note and a single soiled bank note. The reason for this is that the voltage output of the optical sensor when the light incident on it has passed through one note is typically 20% of the output of the optical sensor when light from the emitter has reached the sensor directly (i.e. with no note present at the sensing station), and that the voltage output of the optical sensor when the incident light has passed through two notes is typically only 20% of the output of the sensor when the incident light has passed through one note. For example, if the output of the sensor when no note is present at the sensing station is 5 volts, the output of the sensor with one note present would typically be 1 volt and the output with 2 notes present would typically be 0.2 volt. It is seen that there is a relatively large difference between the sensor outputs for no and one note present, but a relatively small difference between the sensor outputs for 1 and 2 notes present. Thus, there is not a very clearly defined difference as regards the sensor output for single and multiple notes, and the very low sensor output for a multiple note may approach the noise level of an analog to digital (AID) converter included in the circuit used to process the output of the sensor.
Another known system for detecting multiple sheets is disclosed, for example, in EP-B-0344938. This system includes first and second co-operating rollers between which sheets pass as they are fed along a feed path, the first roller having a fixed axis of rotation, and the second roller being resiliently urged towards the first roller so as to enable it to be moved away from the first roller as a single or multiple sheet passes between the rollers. A voltage generating means associated with the second roller produces an output voltage which varies linearly with movement of the second roller towards or away from the first roller, and this output voltage is applied to a processing circuit which determines whether a single or multiple sheet has passed between the rollers. This known system has the disadvantage that it is relatively complicated compared with the known multiple sheet detector system using an optical sensor.